Studying the Darkest World

byPaul GilsteronAugust 12, 2011

A planet orbiting the star GSC 03549-02811, about 750 light years away in direction of the constellation Draco, is showing us a new way of extracting information about a distant system. The planet is a gas giant called TrES-2b, discovered by the Trans-Atlantic Exoplanet Survey in 2006. Studying the star using data from Kepler observations over a span of 50 orbits, David Kipping (Harvard-Smithsonian Center for Astrophysics) and David Spiegel (Princeton University) have detected the faint brightness variations caused by planetary phase changes during its orbits. The light from the planet dims and brightens as it moves through its phases around the star.

“In other words, Kepler was able to directly detect visible light coming from the planet itself,” says Kipping, and what we’ve learned is that TrES-2b is remarkably dark, reflecting less than one percent of the sunlight falling on it. The planet is blacker than any moon or planet in our solar system, as black as coal, or in Kipping’s analogy, “less reflective than black acrylic paint.” That’s quite a contrast with Jupiter, whose ammonia clouds reflect more than a third of incoming sunlight, creating a notably bright object, as anyone who enjoys taking walks at night and looking at the sky can attest.

But TrES-2b is similar to Jupiter only in its mass. This is a world that reaches temperatures of more than 1000 degrees Celsius, making ammonia clouds impossible. It’s no surprise that ‘hot Jupiters’ like this should be dark because of light-absorbing chemicals like vaporized sodium and potassium or gaseous titanium oxide in the atmosphere, all of which would lead to low albedos of a few percent. But a puzzle remains. The team went on to combine the Kepler measurements with Spitzer and ground-based data, all of which suggest, as the paper on this work reports, that there is an extra ‘absorber’ that contributes to the blackness of TrES-2b:

… models with no extra absorber are completely inconsistent with observations, even on the basis of the Kepler data alone. The upshot is that some extra opacity source appears to be required to explain the emergent radiation from this extremely dark world. Owing to this optical opacity, our models that are consistent with the data have thermal inversions in their upper atmosphere…

It’s remarkable to me that using just four months of Kepler photometry, Kipping and Spiegel have been able to detect light from the darkest exoplanet yet found. But the high-precision photometry allowed by a Kepler or CoRoT is now coming into its own, with detections already reported of phase variations for planets like CoRoT-1b, HAT-P-7b, CoRoT-3b and Kepler-7b. Thus a technique that has already been tested and refined through long use in studying eclipsing binary stars is emerging thanks to space-based instruments as a factor in understanding exoplanets, even when, like TrES-2b, they are at the very lowest limits of detectability.

The paper is Kipping and Spiegel, “Detection of visible light from the darkest world,” accepted by Monthly Notices of the Royal Astronomical Society Letters (preprint).

Comments on this entry are closed.

andyAugust 13, 2011, 17:39

It’s getting quite easy to write off the hot Jupiters as uninteresting because they are massively over-represented in the exoplanet catalogue (in fact they are relatively rare but the most successful planet-detection methods are strongly biased towards finding this type of planet). On the other hand it is still clear that there is a lot to learn about these planets, and they show a wide variety of characteristics.

We’ve had this discussion here before (and it’s been mentioned in the comments at space.com) but this planet has some of the properties one would expect of a stellar power-sat, particularly the low albedo. It is probably much too large, and it’s very uneconomical for a power-sat to be spherical, but the phase changes could instead be induced by a flat object that is synchronously rotating. The phase relation would be different in detail though, as it would be at a minimum at quadrature (90 degrees either side of transit), unlike that of a spherical object, whose lightcurve would be at a minimum during transit. Just a thought or two…..

There should be literally HUNDREDS of objects hiding in the Kepler dataset that show similar phase changes, but are NOT transiting. To my knowledge, these haven’t been detected yet (or at least announced) and are probably the last class of EXPECTED objects to yet be detected (ok, not including t!he obvious case of earth-sized objects in the habitable zones of solar type stars!)

@Enzo: the Sudarsky system hasn’t yet been of much relevance to these studies, certainly it isn’t holding up too well in the hot Jupiter domain, and we don’t yet have enough colder transiting planets to test the rest of it.

@coolstar: the slight problem with that hypothesis is that we also have a mass measurement for TrES-2b thanks to the reflex velocity of the star, and it turns out that it is about 1.2 times more massive than Jupiter. This is the kind of value you would expect if it was a planet and not some kind of satellite.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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